Photonic transmitter

ABSTRACT

A photonic transmitter is provided, including a laser source including a first waveguide made of silicon and a second waveguide made of III-V gain material, the waveguides being separated from each other by a first segment of a dielectric layer; and a phase modulator including a first electrode made of single-crystal silicon and a second electrode made of III-V crystalline material, separated from each other by a second segment of the dielectric layer, where a thickness of the dielectric layer is between 40 nm and 1 μm, where a thickness of a dielectric material in an interior of the first segment is equal to the thickness of the dielectric layer, and where a thickness of the dielectric material in an interior of the second segment is between 5 nm and 35 nm, a rest being formed by a thickness of semiconductor material.

The invention relates to a photonic transmitter comprising a lasersource and a modulator. Another subject of the invention is a processfor fabricating this transmitter.

Known photonic transmitters comprise:

a stack comprising a substrate mainly lying in a plane called the “planeof the substrate” and the following layers successively stacked one ontop of the other and each mainly lying parallel to the plane of thesubstrate:

-   -   a first layer located directly on the substrate and comprising        single-crystal silicon encapsulated in a dielectric material,    -   a second layer located directly on the first layer and        comprising a dielectric material,    -   a third layer located directly on the second layer and        comprising a III-V gain material and a doped III-V crystalline        material, which materials are encapsulated in a dielectric        material,        a semiconductor laser source able to generate an optical signal,        this laser source comprising:    -   a first waveguide made of silicon structured in the        single-crystal silicon of the first layer, and    -   a second waveguide made of III-V gain material structured in the        III-V gain material of the third layer, the first and second        waveguides being optically coupled to each other by adiabatic        coupling and being separated from each other to this end by a        first segment of the second layer,        a phase modulator produced on the same substrate and able to        modulate the optical signal generated by the semiconductor laser        source, this modulator comprising:    -   a first electrode made of n- or p-doped single-crystal silicon a        first portion of which is structured in the single-crystal        silicon of the first layer,    -   a second electrode made of doped III-V crystalline material        structured in the doped III-V crystalline material of the third        layer, the doping of this second electrode being of opposite        type to that of the first electrode, said second electrode being        separated from the first electrode by a second segment of the        second layer.

Such a transmitter is for example disclosed in application EP3206079A1.

Prior art is also known from:

WO2018/029414A1, Hyundai Park et Al: “Device and Integration Technologyfor Silicon Photonic Transmitters”, IEEE Journal of Selected Topics inQuantum Electronics, Vol. 17, N°3, Jan. 1, 2011, FR3041116A1,WO2014/155450A1.

The invention described here aims to propose a transmitter having thesame advantages as that of application EP3206079A1 while being simplerto fabricate.

One subject thereof is therefore such a transmitter.

Another subject of the invention is a process for fabricating thesemiconductor photonic transmitter that is one subject of the presentpatent application.

The invention will be better understood on reading the followingdescription, which is given merely by way of nonlimiting example withreference to the drawings, in which:

FIG. 1 is a schematic illustration of a transmitter in vertical crosssection;

FIG. 2 is a partial schematic illustration, in vertical cross section,of details of the transmitter of FIG. 1;

FIG. 3 is a flowchart of a first process for fabricating the transmitterof FIG. 1;

FIGS. 4 to 11 are schematic illustrations, in vertical cross section, ofvarious fabrication states obtained during the implementation of theprocess of FIG. 3;

FIG. 12 is a partial flowchart of a second process for fabricating thetransmitter of FIG. 1;

FIG. 13 is a schematic illustration, in vertical cross section, of afabrication state obtained during the implementation of the process ofFIG. 12;

FIG. 14 is a flowchart of a third process for fabricating thetransmitter of FIG. 1;

FIGS. 15 to 17 are schematic illustrations, in vertical cross section,of the various fabrication states obtained during the implementation ofthe process of FIG. 14.

In these figures, the same references have been used to referenceelements that are the same. In the rest of this description, featuresand functions well known to those skilled in the art are not describedin detail.

Section I: Examples of Embodiments

FIG. 1 shows a transmitter 5 of a phase- and/or amplitude-modulatedoptical signal for transmitting data bits to a receiver by way of anoptical fibre for example. To this end, the transmitter 5 comprises alaser source 7 that emits an optical signal the phase and/or amplitudeof which is then modulated by a system 6 for modulating the phase and/oramplitude of this optical signal.

For example, the wavelength λ_(Li), of the optical signal emitted by thelaser source 7 is comprised between 1240 nm and 1630 nm.

The system 6 may be a system for modulating a phase alone, or amplitudealone or phase and amplitude simultaneously.

Typically, the laser source 7 is a DBR laser (i.e. a distributed Braggreflector laser) or a DFB laser (i.e. a distributed feedback laser).Such laser sources are well known and only details required tounderstand the invention will be described here. For example, forgeneral details and a description of the operation of such lasersources, the reader may refer to the following articles:

B. Ben Bakir et al., “Hybrid Si/III-V lasers with adiabatic coupling”,2011.

B. Ben Bakir, C. Sciancalepore, A. Descos, H. Duprez, D. Bordel, L.Sanchez, C. Jany, K. Hassan, P. Brianceau, V. Carron, and S. Menezo,“Heterogeneously Integrated III-V on Silicon Lasers”, Invited Talk ECS2014.

To simplify FIG. 1 and the following figures, only a hybrid laserwaveguide 200, 220 and a surface grating coupler 8 of the laser source 7have been shown.

Such a coupler 8 is for example described in the following article: F.Van Laere, G. Roelkens, J. Schrauwen, D. Taillaert, P. Dumon, W.Bogaerts, D. Van Thourhout and R. Baets, “Compact grating couplersbetween optical fibers and Silicon-on-Insulator photonic wire waveguideswith 69% coupling efficiency”. It is produced in a layer 3 comprisingsingle-crystal silicon encapsulated in a dielectric material 116.Generally, a dielectric material has an electrical conductivity at 20°C. lower than 10⁻⁷ S/m and, preferably, lower than 10⁻⁹ S/m or 10⁻¹⁵S/m. In addition, in the case of the dielectric material 116, itsrefractive index is strictly lower than the refractive index of silicon.For example, in this embodiment, the dielectric material 116 is silicondioxide (SiO₂).

The layer 3 mainly lies in a horizontal plane. The layer 3 is locateddirectly on a rigid substrate 44. In the layer 3, the single-crystalsilicon is located in a given horizontal plane parallel to the plane ofthe substrate 44. Here, the single-crystal silicon of the layer 3 isalso mechanically and electrically insulated from the substrate 44 bythe dielectric material 116. For example, the thickness ofsingle-crystal silicon in the layer 3 is comprised between 100 nm and800 nm. In this example, the thickness of the single-crystal silicon inthe layer 3 is equal to 65 nm or 150 nm or 300 nm or 500 nm.

In FIG. 1 and in the following figures, the horizontal is represented bythe directions X and Y of an orthogonal coordinate system. The directionZ of this orthogonal coordinate system represents the verticaldirection. Below, terms such as “upper”, “lower”, “above”, “below”,“top” and “bottom” are defined with respect to this direction Z. Theterms “left” and “right” are defined with respect to the direction X.The terms “front” and “back” are defined with respect to the directionY.

FIG. 1 shows the elements of the transmitter 5 in cross-section in avertical plane parallel to the directions X and Z.

The hybrid laser waveguide 200, 220 consists of a waveguide 200 made ofa III-V gain material and a waveguide 220 made of single-crystalsilicon. Generally, the waveguide 200 is used to generate and amplify anoptical signal in the interior of an optical cavity of the laser source7. Here, to this end, it is produced in a layer 36 comprising a III-Vgain material encapsulated in a dielectric material 117. For example,the material 117 is silicon dioxide or silicon nitride. This layer 36lies horizontally directly on a layer 20 made of dielectric material.The layer 20 itself lies horizontally directly on the layer 3, on theside of this layer 3 opposite the substrate 44.

The substrate 44 lies horizontally in a plane called the “plane of thesubstrate”. In this example embodiment, the substrate 44 is a carrierthe thickness of which is typically larger than 200 μm or 400 μm. Forexample, the substrate 4 is a carrier made of silicon.

The thickness e₂₀ of the layer 20 is typically comprised between 40 nmand 1 μm or 500 nm and, preferably, between 50 nm and 150 nm or between50 nm and 140 nm. By way of illustration, here, the thickness e₂₀ isequal to 85 nm or 100 nm.

The layer 36 typically comprises a doped lower sublayer 30, a stack 34of quantum wells or quantum dots in a quaternary material and an uppersublayer 32 doped with a dopant of opposite type to that of the sublayer30. The sublayers 30 and 32 are for example here made of p- or n-dopedsingle-crystal InP alloy. In this case, the stack 34 is, for example, astack in alternation of sublayers made of InGaAsP or of AlGaInAs interalia.

In FIG. 1, only a strip 33, a stack 233 and a strip 234 produced,respectively, in the sublayer 30, the stack 34 and the sublayer 32 havebeen shown. This superposition of the strip 33, of the stack 233 and ofthe strip 234 forms the waveguide 200.

The waveguide 200 also comprises:

contacts 243G and 243D making mechanical and electrical contact directlywith the strip 33 and located, to the left and to the right of the stack233, respectively, anda contact 244 making mechanical and electrical contact directly with thestrip 234.

These contacts 243G, 243D and 244 allow an electrical current to beinjected into the waveguide 200 made of III-V gain material between thecontacts 243G, 243D and the contact 244.

The waveguide 220 is made of silicon and structured in thesingle-crystal silicon of the layer 3. This waveguide 220 extends underthe strip 33. In FIG. 1, the waveguide 220 is shown, by way ofillustration, in the particular case where the propagation direction ofthe optical signal in the interior of this waveguide is parallel to thedirection Y. Here, the waveguide 220 is a rib waveguide. Thus, the crosssection of this waveguide, parallel to the XZ plane, has a central rib222 from which extend, on each side, parallel to the direction X,thinner lateral arms 223G and 223D. Here, the waveguide 220 is separatedfrom the strip 33 only by a segment 20A of the layer 20.

The waveguide 220 is optically connected to the waveguide 200 viaadiabatic coupling. For a detailed description of adiabatic coupling,the reader may refer to the following article: Amnon Yariv et al.,“Supermode Si/III-V hybrid Lasers, optical amplifiers and modulators:proposal and analysis” Optics Express 9147, vol. 14, No. 15, 23, Jul.2007.

The characteristics of the optical coupling between the waveguide 220and the waveguide 200 notably depend:

on the dimensions of the waveguide 220 and, in particular, the thicknesse₂₂₂ of the central rib 222, andon the thickness e_(20A) of dielectric material in the segment 20A ofthe layer 20 interposed between the waveguides 200 and 220.

It is therefore important for the thicknesses e₂₂₂ and e_(20A) to beable to be adjusted independently of the dimensions of the otherphotonic components produced on the same substrate 44. Here, to optimisethe operation of the laser source 7, the thickness e_(20A) is largerthan 40 nm and, typically, comprised between 40 nm and 1 μm or between40 nm and 500 nm. Here, the thickness e_(20A) is equal to the thicknesse₂₀ of the layer 20. With such a choice of the thickness e₂₀A, thecoupling between the waveguides 200 and 220 is adiabatic. Under theseconditions, preferably, only the waveguide 220 has tapered ends in orderto ensure a good exchange of optical power between the waveguides 200and 220.

Again to optimise the operation of the laser source 7, the thicknesse₂₂₂ is larger than or equal to 220 nm or 300 nm. For example, here, thethickness e₂₂₂ is equal to 500 nm.

To modulate the phase or amplitude of the optical signal, the system 6comprises at least one phase modulator and, often, at least onephase-tuning device. For example, the system 6 is a Mach-Zehnderinterferometer in which the modulator and the phase-tuning device arearranged in one of the branches of this interferometer in order tomodulate the amplitude and/or the phase of the optical signal generatedby the laser source 7. The structures of a Mach-Zehnder interferometerand of a phase-tuning device are well known and are not described indetail here. The phase-tuning device is for example the same as thatdescribed in application EP3206079. Therefore, to simplify FIG. 1, onlya phase modulator 100 has been shown.

The modulator 100 allows the phase of the optical signal to be rapidlymodified. To this end, the modulator 100 is here an electro-opticmodulator (EOM) and, more precisely, a hybrid capacitive modulator. Ittherefore comprises two electrodes 120 and 130 that are located facingeach other and that form a capacitor.

The electrode 120 is made of doped single-crystal silicon. It is atleast partially structured in the single-crystal silicon of the layer 3.It extends, in the direction X, from a near end 12 to a far end 11. Italso extends in the direction Y.

Here, the far end 11 is more highly doped than the near end 12. Forexample, the dopant concentration in the far end 11 is comprised between10¹⁹ and 10 ²¹ atoms/cm³. The dopant concentration in the near end 12 isfor example comprised between 10¹⁷ and 10¹⁹ atoms/cm³.

The electrode 130 is made of doped III-V crystalline material that isdoped the opposite type to the electrode 120. Here, it is made from InP.The dopant concentration of the electrode 130 is for example comprisedbetween 10¹⁷ and 2×10¹⁸ atoms/cm³ or between 10¹⁷ and 2×10¹⁹ atoms/cm³.

The electrode 130 extends, parallel to the direction X, from a near end32 to a far end 31. The electrode 130 also extends in the direction Y.It is located directly on the layer 20.

The near end 32 is located facing the near end 12 and separated fromthis near end 12 only by a segment 20B of the layer 20 interposedbetween these near ends. With respect to a vertical plane parallel tothe directions Y and Z and passing through the near ends 12 and 32, thefar end 31 is located on one side of this plane whereas the other farend 11 is located on the other side. The far ends 11 and 31 aretherefore not facing.

The zone 34, which extends vertically from the far end 31 to thesubstrate 44, comprises solely solid dielectric materials. Here, it is aquestion of the dielectric material 116 and of the dielectric materialof the layer 20. By virtue of this, the parasitic capacitance betweenthis end 31 and the substrate 44 is greatly decreased. This zone 34 is,for example, identical to that described in application EP3206079.

The superposition, in the direction Z, of the near end 12, of thesegment 20B of the layer 20 and of the near end 32 is dimensioned toform a waveguide 70, capable of guiding, in the direction Y, the opticalsignal generated by the laser source 7. The waveguide 70 is typicallyoptically connected to the laser source 7 by way of other waveguides andof other couplers structured in the layer 3. To simplify FIG. 1, theseother waveguides and other couplers have not been shown.

The modulator 100 also comprises two contacts 21 and 22, makingmechanical and electrical contact directly with the far ends 11 and 31,respectively. These contacts 21 and 22 are connected to a voltage sourcethat is controllable depending on the data bit or bits to be transmittedby the transmitter 5.

FIG. 2 shows in more detail the structure of the electrode 120 and ofthe waveguide 220. In this figure, the wavy lines indicate that portionsof the transducer 5 have not been shown. In addition, to simplify thefigure, the contacts 21, 22, 243G and 243D have not been shown.

The central rib 222 of the waveguide 220 comprises a central portion222A on the lower face of which is directly deposited an additionalthickness 222B of single-crystal silicon in order to achieve the desiredthickness e₂₂₂. The limit between the central portion 222A and theadditional thickness 222B has been represented in FIG. 2 by a dashedhorizontal line. In this embodiment, the additional thickness 222B isalso made of single-crystal silicon.

For example, here, the thickness e_(222A) of the central portion 222A isequal to 300 nm and the thickness e_(222B) of the additional thickness222B is equal to 200 nm. Thus, the thickness e₂₂₂ of the rib 222 isequal to 500 nm. In this embodiment, the thickness e₂₂₃ of the lateralarms 223D and 223G is equal to the thickness e_(222A), i.e. to 300 nm.

The far end 11 extends continuously from the interface between thelayers 3 and 20 to a lower face buried in the interior of the layer 3.The thickness of the far end 11 is denoted e₁₁. Here, the thickness e₁₁is equal to 300 nm. In this embodiment, the near end 12 and far end 11are mechanically and electrically connected to each other by anintermediate portion 576 the thickness e₅₇₆ of which is strictly smallerthan the thickness e₁₁ and than the thickness e₁₂ of the near end 12.The thickness e₅₇₆ is chosen so as to prevent or limit the propagationof the optical signal in the far end 11. For example, the thickness e₅₇₆is smaller than or equal to e₁₂/2. Here, the difference between thethicknesses e₅₇₆ and e₁₁ is typically larger than 50 nm or 100 nm.Similarly, the difference between the thicknesses e₅₇₆ and e₁₂ istypically larger than 50 nm or 100 nm. To limit the number of etchinglevels, the thickness e₁₁ is equal to the thicknesses e_(222A). Here,the thickness e₅₇₆ is equal to 65 nm. This configuration of theelectrode 120 also allows the capacitance of the modulator to be bettercontrolled.

The near end 12 is produced in two portions:

a first portion 12A structured solely in the single-crystal silicon ofthe layer 3, anda second portion 12B made of single-crystal silicon produced in theinterior of the segment 20B of the layer 20 and located directly on theportion 12A. In FIG. 2, the limit between the portions 12A and 12B hasbeen represented by a dashed horizontal line. The thickness of theportion 12B is denoted e_(12B). This thickness is measured from theinterface between the layers 3 and 20 to the upper face of the portion12B buried in the interior of the layer 20.

In the interior of the segment 20B, because of the presence of theportion 12B, the thickness e_(20B) of the dielectric material of thelayer 20 that mechanically and electrically isolates the near ends 12and 32 is smaller. Here, the thickness e_(12B) is chosen so that theremaining thickness e_(20B) of dielectric material is quite thin inorder to obtain a good capacitive coupling between the electrodes 120and 130. To this end, the thickness e_(20B) is comprised between 5 nmand 35 nm. For example, the thickness e_(20B) is here equal to 10 nm.The thickness e_(12B) of the portion 12B is related to the thicknessese_(20B) and e₂₀ by the following relationship: e_(12B)=e₂₀−e_(20B).

Here, to limit the number of etching levels of the single-crystalsilicon of the layer 3, the thickness of the portion 12A is chosen equalto the thickness e₅₇₆.

In this embodiment, the thickness e₃₂ of the near end 32 of theelectrode 130 is chosen to optimise the ratio of the optical phase shiftto the optical loss. To do this, for example, various values of thethickness e₃₂ are simulated experimentally in order to determine whichof these values allows this ratio to be optimised. For example, in thepresent case where the thicknesses e₁₂ and e_(20B) are equal to 140 nmand 10 nm, respectively, it has been determined experimentally that thethickness e₃₂ that optimises this ratio is comprised between 80 nm and100 nm. Here, the thickness e₃₂ is chosen equal to 90 nm.

One possible way in which the transmitter 5 may operate is thefollowing. The laser source 7 generates an optical signal. At least oneportion of this optical signal is directed to a Mach-Zehnderinterferometer at least one of the branches of which comprises themodulator 100. This portion of the optical signal is therefore guided bythe waveguide 70 before being recombined with another portion of theoptical signal that is guided by the other branch of the Mach-Zehnderinterferometer in order to form the modulated optical signal.

A process for fabricating the transmitter 5 will now be described withreference to FIGS. 3 to 11. FIGS. 4 to 11 show various states offabrication of the transmitter 5 in vertical cross section parallel tothe directions X and Z. To simplify these figures and the description ofthis process, the conventional steps and operations used to fabricateoptical components other than those shown in FIG. 1 and necessary to theoperation of the transmitter 5 have been omitted.

In a step 500, the process starts with provision of a substrate 4 (FIG.4). Here, this substrate 4 is a silicon-on-insulator (SOI) substrate.The substrate 4 comprises stacked directly on top of one another in thedirection Z:

a carrier 1 made of silicon, of thickness larger than 400 μm or 700 μmconventionally,a buried layer 2 made of silicon dioxide, anda layer 43 made of single-crystal silicon that, at this stage, has notyet been etched or encapsulated in a dielectric material.

The thickness of the layer 2 is larger than or equal to the thicknesse_(12B). Here, the thickness of the layer 2 is therefore larger than 75nm or 90 nm. For example, the thickness of the layer 2 is larger than500 nm or 1 μm and, generally, smaller than 10 μm or 20 μm. In thisexample embodiment, the thickness of the layer 2 is equal to 800 nm.

The thickness of the layer 43 is here equal to the thickness e₁₁ ore_(222A). It is therefore equal to 300 nm. In this embodiment, thethickness of the layer 43 is in contrast smaller than the desiredthickness e₂₂₂.

In a step 502, the layer 3 is produced. To do this, the layer 43 isetched in order to structure the various optical-component portionslocated in the interior of the layer 3 and the thicknesses of which aresmaller than or equal to the thickness of the layer 43. Thus, in thisstep 502, the arms 223D, 223G, the central portion 222A, the far end 11,the intermediate portion 576, the portion 12A of the near end 12 and thesurface grating coupler 8 are structured in the layer 43 ofsingle-crystal silicon.

For example, in an operation 514, the layer 43 undergoes a partiallocalised first etch (FIG. 5) in order to thin the thickness of thesilicon in the locations required to produce the surface grating coupler8, the intermediate portion 576 and the portion 12A of the near end 12.At the end of the operation 514, the thinned regions have a thicknessequal to the thickness e₅₇₆.

In contrast, in this operation 514, other regions that are referred toas “non-thinned” regions are not etched and preserve their initialthickness. In particular, these non-thinned regions are located in thelocation of the central portion 222A of the rib 222, of the arms 223Dand 223G and in the location of the far end 11.

In an operation 516, a complete localised etch of the layer 43 iscarried out (FIG. 6). Contrary to the partial etch, the complete etchcompletely removes the thickness of silicon of the layer 43 in theunmasked regions to which it is applied. In contrast, masked regionsprotect the layer 43 from this complete etch. This complete etch iscarried out so as to structure, simultaneously in the layer 43, the arms223D, 223G, the central portion 222A, the far end 11, the intermediateportion 576 and the portion 12A of the near end 12 and the surfacegrating coupler 8. To this end, only the regions corresponding to thesevarious elements are masked. At the end of this step, the state shown inFIG. 6 is obtained.

In an operation 518, the layer 43 of single-crystal silicon, which wasstructured in the preceding steps, is encapsulated in the dielectricmaterial 116 (FIG. 7). At the end of the operation 518, the thickness ofthe layer 3 is equal to the thickness e₂₂₂, i.e. equal to 500 nm.

Next, in an operation 520, the additional thickness 222B is formed onthe central portion 220A already structured in the preceding operations.Here, the additional thickness 222B is formed by implementing what isknown as the damascene process. This process is for example describedwith reference to FIG. 3 of the following article: P. Dong et al.:“Novel integration technique for silicon/III-V hybrid laser”, OpticsExpress 22(22), Vol. 22, N°22, pp. 26861, 2014. To summarise, itconsists in etching into the material 116 a trench the bottom of whichopens onto the central portion 222A then in depositing or growingepitaxially single-crystal silicon in the interior of this cavity.Lastly, a polishing operation allows the excess of single-crystalsilicon that was deposited or that was grown above and/or on the trenchto be removed.

To finish production of the layer 3, in an operation 522, a newencapsulating operation is implemented in order to encapsulate theadditional thickness 222B in the interior of the material 116.

The upper face of the material 116 is then prepared for bonding, forexample direct or molecular bonding, i.e. bonding without addition ofmaterial. For example, the upper face of the material 116 is polishedusing a process such as chemical-mechanical polishing (CMP).

In a step 526, the upper face of the substrate 4, i.e. at this stage thepolished face of the material 116, is then bonded to the exterior faceof the substrate 44 (FIG. 9), for example, by direct bonding. Thesubstrate 44 has already been described with reference to FIG. 1.

Next, in a step 527, the layer 20 is fabricated on the layer 3. To dothis, in an operation 528, the carrier 1 is removed, and the layer 2 ispartially thinned in order to leave behind only a thin sublayer 530(FIG. 10) of silicon dioxide on the layer 3. The thickness of thesublayer 530 is equal to the thickness e_(12B). Here, its thickness istherefore equal to 75 nm or 90 nm.

In an operation 532, the portion 12B of the far end 12 is formed on theportion 12A. For example, the portion 12B is produced by implementing adamascene process as in the operation 520. In this case, the trench isetched in the sublayer 530 and opens onto the upper face of the portion12A already produced. The thickness of the portion 12B thus formed isequal to the thickness of the sublayer 530 and therefore equal to 75 nmor 90 nm. The state shown in FIG. 11 is then obtained.

In an operation 534, localised doping operations are implemented inorder to obtain the various doping levels desired for the near and farends 11, 12 of the electrode 120. Since these localised dopingoperations are conventional, they are not described here. Preferably,the electrode 120 is p-doped. Here, this doping is obtained by ionimplantation of boron.

In an operation 540, the portion 12B is encapsulated in a secondsublayer of dielectric material in order to finalise the production ofthe layer 20. For example, this dielectric material is silicon dioxide.Thus, at the end of this operation, the portion 12B is covered with athickness e_(20B) of silicon dioxide and the thickness of the segment20A of the layer 20 now has a thickness e_(20A) equal to the desiredthickness. In this operation 540, the upper face of the layer 20 is alsoprepared for bonding, for example direct or molecular bonding. Thus,typically, the upper face of the layer 20 is polished using a processsuch as chemical-mechanical polishing (CMP).

Lastly, in a step 541, the layer 36 is fabricated.

In an operation 542, the electrode 130 and the waveguide 200 areproduced on the upper face of the layer 20. Here, the electrode 130 andthe waveguide 120 are produced by direct bonding two separate stacksmade of III-V materials, one being used to produce the electrode 130 andanother being used to produce the waveguide 200. The use of two separatestacks made of III-V material allows the thickness of the electrode 130to be chosen independently of the thickness of the strip 33. Productionof the electrode 130 and of the waveguide 200 by bonding and structuringstacks of III-V material is conventional and is not described in moredetail here. For example, the interested reader may consult thefollowing articles for examples of such techniques:

H. Duprez et al.: “1310 nm hybrid InP/InGaAsP on silicon distributedfeedback laser with high side-mode suppression ratio”, Optic Express23(7), pp. 8489-8497 (2015),J.-H. Han et al.: “Efficient low-loss InGaAsP/Si hybrid MOS opticalmodulator”, Nature Photonics 11, pp. 486 (2017).

In an operation 544, the electrode 130 and the waveguide 200 areencapsulated in the dielectric material 117. The layer 36 comprising,encapsulated in the dielectric material 117, the III-V gain material andthe doped III-V material, is then obtained. Lastly, the contacts 21, 22,243G and 243D are produced. The transmitter 5 such as shown in FIG. 1 isthen obtained.

FIG. 12 shows a flowchart of a second possible process for fabricatingthe transmitter 5. The process of FIG. 12 is identical to the process ofFIG. 3 except that:

the initial thickness of the layer 43 made of single-crystal silicon ofthe substrate 4 provided in step 500 is equal to 500 nm and no longer to300 nm andthe step 502 producing the layer 3 has been replaced by a step 602.

Step 602 is identical to step 502 except that an additional localisedetching operation 604 has been introduced before the localised etchingoperation 514. In the operation 604, the layer 43 undergoes a partiallocalised first etch in order to thin the thickness of the siliconeverywhere except in the location of the additional thickness 222B. Atthe end of the operation 604, the thickness of the layer 43 is equal to300 nm except in the location of the additional thickness 222B where itsthickness is equal to 500 nm. Next, the operations 514 and 516 arecarried out while taking care not to thin the thickness of the layer 43in the location of the additional thickness 222B. Thus, at the end ofthe operations 604, 514 and 518, the state shown in FIG. 13 is obtained.In this embodiment, the additional thickness 222B is achieved by etchingthe layer 43. Thus, the operations 518 and 520 are omitted.

Subsequently, the steps of this process are identical to those alreadydescribed with reference to FIG. 3. To simplify FIG. 12, they have notbeen shown and have been replaced by a dashed line.

FIG. 14 shows a third possible process for fabricating the transmitter5. This process starts with the production of the layer 20 before theproduction of the layers 3 and 36. In this process, steps and operationsalready described with reference to FIG. 3 are not described again.

After step 500, this process comprises a step 700 of producing the layer20. For example, the step 700 starts with an operation 702 in which theportion 12B of the electrode is produced. Here, this operation 702 isidentical to the operation 532. To this end, initially, in step 700, anoxide layer 704 (FIG. 15) the thickness of which is equal to thethickness e_(12B) is produced on the upper face of the substrate 4. Atthe end of the operation 702, the state shown in FIG. 15 is obtained.

Next, in an operation 706, the produced portion 12B is encapsulated insilicon dioxide. For example, this step is identical to the operation540 of the process of FIG. 3. Thus, at the end of step 700, the layer 20immediately above the layer 43 is obtained.

Next, in a step 710, the layer 3 is produced. To do this, in anoperation 712, the upper face of the layer 20 is bonded, for example bydirect bonding, to a substrate 714 made of amorphous silicon. The stateshown in FIG. 16 is then obtained.

In an operation 716, the carrier 1 and the layer 2 are removed in orderto uncover the layer 43 made of single-crystal silicon.

Next, the following operations are carried out:

the localised etching operation 514, thenthe localised complete etching operation 516, thenthe encapsulating operation 518, thenthe operation 520 of forming the additional thickness 222B, thenthe operation 522 of encapsulating the formed additional thickness 222B.

At the end of step 710, the state shown in FIG. 17 is obtained.

The upper face of the material 116 is then prepared for bonding, forexample direct bonding.

The step 526 of bonding the upper face of the layer 36 to the upper faceof the substrate 4 is then executed.

In a step 720, the substrate 714 is then removed in order to uncover theface of the layer 20.

Lastly, step 514 of fabricating the layer 36 on the layer 20 isexecuted.

Section II: Variants Variants of the Electrode 120

The intermediate portion 576 may be separated from the layer 20 by arecess filled with a dielectric material. Typically, the recess isfilled with the material 116. The bottom of this recess is essentiallyhorizontal and spaced apart from the layer 20 by a depth p₅₇₈. The depthp₅₇₈ is typically larger than 50 nm or 100 nm. Here, the depth p₅₇₈ isequal to 65 nm. This configuration of the intermediate portion allowsthe width of the waveguide 70 to be better controlled. Such aconfiguration of the intermediate portion 576 and its fabrication aredescribed in application EP3206079. It is therefore not described inmore detail here.

The intermediate portion 506 may also be omitted.

In one variant, the doping of the near end 12 and of the far end 11 isthe same. Thus, during the fabrication of the electrode 120, only asingle step of doping the layer 43 made of single-crystal silicon isrequired.

The electrode 120 may also be n-doped. Such n-doping is for exampleobtained by ion implantation of phosphorus. In this case, the electrode130 is p-doped.

The portions 12A and 12B of the far end 12 are not necessarily made fromthe same semiconductor material. For example, the portion 12B may bemade from SiGe alloy or be formed by a superposition of layers made ofsilicon and of SiGe alloy. The portion 12B may also be made fromamorphous silicon. For example, this additional thickness of amorphoussilicon may be deposited on the single-crystal silicon using one of theprocesses described in the following articles:

T. Ferrotti et al., “O-band III-V-on-amorphous-silicon lasers integratedwith a surface grating coupler”, IEEE Phot. Technol. Lett. 28(18)pp.1944, 2016,B. Szelag et al., “Hybrid III-V/Si DFB laser integration on a 200 mmfully CMOS-compatible silicon photonics platform”, in Proc. of ElectronDevices Meeting (IEDM), 2017 IEEE International, pp. 24.1, 2017.

Variants of the Electrode 130

The III-V material used to produce the electrode 130 may be different.For example, it may be a question of the alloy InP, the alloy GaAs, thealloy InGaAsP, or AlGaInAs or of a superposition of a plurality of thesealloys.

In another embodiment, the electrode 130 is made from a III-V materialdifferent from that used to produce the strip 33.

The thickness e₃₂ may be chosen to be different. For example, as avariant, the thicknesses e₁₂ and e₃₂ of the near ends 12 and 32,respectively, are chosen so that the point at which the maximum strengthof the optical field of the optical signal that propagates through thewaveguide 70 is located is as close as possible to the thickness ofdielectric material located in the interior of the segment 20B.Preferably, this point M is located at the centre of the thicknesse_(20B) of dielectric material of the segment 20B. Specifically, it isat the interfaces between the near ends 12, 32 and the dielectricmaterial of the segment 20B that the charge carrier density is maximumwhen a potential difference is present between the near ends 11 and 31.This therefore improves the effectiveness of the modulator 100. Forexample, to this end, in the case where the refractive indices of thenear ends 12 and 32 are close to each other, the thicknesses e₁₂ and e₃₂are also chosen to be substantially equal. For example, the thicknesse₁₂ is comprised between 0.5e₃₂ and 1.5e₃₂, and, preferably, between0.7e₃₂ and 1.3e₃₂.

Variants of the Laser Source

As a variant, the additional thickness 222B is omitted. In this case,the maximum thickness of the waveguide 220 is equal to the thickness ofthe layer 43.

As a variant, the additional thickness 222B is made of amorphous siliconand not of single-crystal silicon. In this case, the additionalthickness is deposited on the single-crystal silicon, not by epitaxialgrowth but by deposition.

Other III-V gain materials may be used to produce the laser source 7.For example, the layer 36 is formed from the following stack from thebottom upwards:

a lower sublayer made of n-doped GaAs,sublayers containing AIGaAs quantum dots, or AIGaAs quantum wells, andan upper sublayer made of p-doped GaAs.

The waveguide 220 may have a so-called “strip” configuration, i.e. thelateral arms 223G and 223D are omitted, or any other configurationcapable of guiding an optical signal.

Other Variants of the Modulator

The modulator 100 may also be a ring modulator. To this end, thewaveguide 70 loops back onto itself in order to form a ring waveguide inwhich the charge carrier density may be modified depending on thepotential difference applied between the contacts 21 and 22. Typically,this ring waveguide is connected to a waveguide through which theoptical signal to be modulated propagates via evanescent coupling. Thewaveguide 70 may also form only a limited segment of the ring waveguide.

In another embodiment, the modulator is used to modulate the intensityof the optical signal that passes therethrough. Specifically, amodification of the charge carrier density in the waveguide 70 alsomodifies the intensity of the optical signal passing therethrough.

Other dielectric materials are possible for the material 116 and thelayer 20. For example, it may be a question of silicon nitride,aluminium nitride, an electrically insulating polymer, or Al₂O₃. Inaddition, in the case of the layer 20, its refractive index is notnecessarily lower than that of silicon.

In another variant, the layer 20 is completely removed wherever it isnot indispensable to the operation of the transmitter. For example, itis completely removed outside of the segments 20A and 20B.

Whatever the embodiment, it is possible to invert the p- and n-dopedregions.

As a variant, some or all of the contacts are produced, not through thematerial 117, but through the substrate 44. In this case, with respectto what was shown in the preceding figures, one or more electricalcontacts emerge under the substrate.

The substrate 44 may be made from a material other than silicon.

As a variant, the waveguides 70, 220 are curved. In this case, theconfiguration of the various elements optically coupled to thesewaveguides is adapted to the radius of curvature of these waveguides.

Variants of the Fabrication Process

Step 502 may comprise other operations in addition to the etchingoperations for structuring the optical-component portions in the layer43 made of single-crystal silicon. For example, it may in additioncomprise operations for doping the layer 43 locally. For example, theselocalised doping operations may be used to obtain the doping levelrequired for the end 11 of the electrode 120. In this case, step 534must be adapted accordingly.

As a variant, the electrode 130 and the waveguide 200 are produced fromthe same stack of III-V material bonded to the upper face of the layer20. This stack is then structured to obtain the electrode 130 and thewaveguide 200.

The insulating layer 20 may be obtained via different fabricationprocesses. For example, in the operation 528, the layer 2 is completelyremoved so as to uncover the layer 3, then the sublayer 530 is depositedon the uncovered layer 3. In this case, optionally, the dielectricmaterial of the layer 20 may be different from the material 116. Forexample, it may be a question of a dielectric material such as anelectrically insulating polymer or Al₂O₃. After the complete removal ofthe layer 2, it is also possible to produce the layer 20 by oxidisingthe surface of the layer 3.

The operation 540 of encapsulating the portion 12B may be carried outdifferently. For example, before the electrode 130 is bonded, thesublayer 30 made of III-V material is oxidised to form, after thebonding, the thickness e_(20B) of oxide located above the portion 12B ofthe electrode 120.

In another variant, the localised complete second etch is replaced by auniform etch of all the surface of the layer 3 in order to convert thenon-thinned regions into thinned regions and to completely remove thethinned regions.

Section III: Advantages of the Described Embodiments

The thickness of dielectric material larger than 40 nm in the segment20A allows the waveguide 220 and the waveguide 200 to be coupledadiabatically rather than evanescently. Adiabatic coupling is simpler toachieve than evanescent coupling. Advantageously, in the case ofadiabatic coupling, it is not necessary to structure tapers in thewaveguide 200 made of III-V material. In contrast, in the case ofevanescent coupling, tapers must be structured in the waveguide 200.Now, depending on its thickness and the crystal orientations,structuring tapers in a waveguide made of III-V material is a complexoperation to carry out. Thus, the fact that the thickness of dielectricmaterial between the waveguides 200 and 220 is larger than 40 nmsimplifies the fabrication of the transmitter. In particular, for thisreason, the transmitter 5 is simpler to fabricate than that described inapplication EP3206079A1.

The fact that the thickness of dielectric material between the twoelectrodes 120 and 130 is smaller than 35 nm allows a hybrid capacitivemodulator to be produced. If the thickness of dielectric materialbetween the two electrodes 120 and 130 were larger, only anelectro-absorption modulator (EAM) would be producible. Now, hybridcapacitive modulators have a passband that is much wider than is thecase with electro-absorption modulators. Hybrid capacitive modulatorsare also advantageous with respect to non-hybrid capacitive modulators,in which the electrode 130 is also made of silicon. Specifically, hybridcapacitive modulators allow phase changes to be obtained 10 times to 100times more effectively than with a non-hybrid capacitive modulator.Thus, the described transmitters have a performance that is equal to orbetter than that of the transmitter of application EP3206079A1.

Lastly, the fact that the thickness of dielectric material between thewaveguides 200 and 220 and between the electrodes 120 and 130 isdifferent, allows a transmitter to be obtained that has, at the sametime, the various above advantages, i.e. that is simple to fabricatewhile incorporating, on the same substrate, a laser source and a hybridcapacitive modulator, and while achieving, both for the laser source andfor the hybrid capacitive modulator, an optimised performance.

The fabrication processes described here allow the thickness of thedielectric material in the segment 20A to be adjusted independently ofthe thickness of dielectric material in the segment 20B of the layer 20.The described processes therefore allow, on the same substrate, both asemiconductor laser source and a hybrid capacitive modulator to befabricated.

The fabrication processes described here in particular have thefollowing advantages:

They allow the thickness of the layer 20 to be precisely controlled anda layer 20 that is particularly planar to be obtained because it isproduced on the side of the layer 3, which has the same leveleverywhere, this simplifying the bonding of the stack made of III-Vmaterial used to produce the electrode 130 and the waveguide 200.They allow the thickness of the electrode 120 to be precisely adjustedindependently of the thickness of the waveguide 220 and, more generally,independently of the thickness of the layer 43 made of single-crystalsilicon. This is particularly useful because, generally, to improve theoperation of the laser source 7, it is necessary for the thickness ofthe waveguide 220 to be quite large, i.e. here of about 500 nm. Incontrast, to improve the operation of the modulator 100, as explainedabove, the thickness of the electrode 120 and, in particular, of itsnear end 12 is generally smaller than the thickness e₂₂₂.This process allows the formation of a parasitic capacitance under thefar end 31 to be avoided and therefore allows a rapider operation of themodulator 100.

1.-10. (canceled)
 11. A photonic transmitter, comprising: a stackcomprising a substrate and the following layers successively stacked oneon top of the other and each mainly lying parallel to a plane of thesubstrate: a first layer disposed directly on the substrate andcomprising single-crystal silicon encapsulated in a dielectric material,a second layer disposed directly on the first layer and comprising adielectric material, and a third layer disposed directly on the secondlayer and comprising a III-V gain material and a doped III-V crystallinematerial, wherein the III-V gain material and the doped III-Vcrystalline material are encapsulated in a dielectric material; asemiconductor laser source configured to generate an optical signal, thesemiconductor laser source comprising: a first waveguide made of siliconstructured in the single-crystal silicon of the first layer, and asecond waveguide made of III-V gain material structured in the III-Vgain material of the third layer, the first and second waveguides beingoptically coupled to each other by adiabatic coupling and beingseparated from each other by a first segment of the second layer,wherein in an interior of the first segment of the second layer, athickness of the dielectric material is equal to a thickness of thesecond layer; and a phase modulator produced on the substrate andconfigured to modulate an optical signal generated by the semiconductorlaser source, the phase modulator comprising: a first electrode made ofn-doped or p-doped single-crystal silicon, a first portion of which isstructured in the single-crystal silicon of the first layer, and asecond electrode made of doped III-V crystalline material structured inthe doped III-V crystalline material of the third layer, a dopant of thesecond electrode being of opposite type to that of the first electrode,the second electrode being separated from the first electrode by asecond segment of the second layer, wherein: the thickness of the secondlayer is comprised between 40 nm and 1 μm, and in an interior of thesecond segment of the second layer, a thickness of the dielectricmaterial is comprised between 5 nm and 35 nm, a rest of the thickness ofthe second layer in the interior of the second segment being formed by athickness of semiconductor material directly disposed on the firstportion of the first electrode and which forms a second portion of thefirst electrode, the thickness of semiconductor material being chosenfrom the group consisting of a thickness of silicon, a thickness of aSiGe alloy, and a superposition of thicknesses made of silicon and madeof the SiGe alloy.
 12. The photonic transmitter according to claim 11,wherein: the doped III-V crystalline material of the third layer liesdirectly on the second layer, the first electrode of the phase modulatorextends, in a transverse direction parallel to the plane of thesubstrate, from a near end to a far end, and extends longitudinally in apropagation direction of the optical signal, the second electrode of thephase modulator extends, in the transverse direction, from a near endlocated facing the near end of the first electrode to a far end locatedon a side opposite to the far end of the first electrode with respect toa plane perpendicular to the plane of the substrate and passing throughthe near ends, the second electrode also extending longitudinally in thepropagation direction of the optical signal, and the phase modulatorfurther comprises contacts making direct mechanical and electricalcontact with, respectively, the far ends of the first and the secondelectrodes in order to electrically connect the first and the secondelectrodes to different electrical potentials so as to modify a chargecarrier density in an interior of a third waveguide formed bysuperposition of the near ends of the first and the second electrodesand the second segment of the second layer interposed between the nearends.
 13. The photonic transmitter according to claim 12, wherein amaximum thickness of the near end of the first electrode is strictlysmaller than a maximum thickness of the first waveguide.
 14. Thephotonic transmitter according to claim 12, wherein a maximum thicknessof the near end of the first electrode is comprised between 0.7e₃₂ and1.3e₃₂, where e₃₂ is a maximum thickness of the near end of the secondelectrode.
 15. The photonic transmitter according to claim 11, whereinthe second electrode is made from a III-V material chosen from the groupconsisting of an InP alloy, a GaAs alloy, an InGaAsP alloy, and asuperposition of a plurality of alloys of this group.
 16. The photonictransmitter according to claim 12, wherein the phase modulator furthercomprises a zone solely composed of one or more solid dielectricmaterials that extend: in the direction perpendicular to the plane ofthe substrate, from the far end of the second electrode to thesubstrate, and in the transverse direction and in the propagationdirection of the optical signal, under an entirety of the far end of thesecond electrode.
 17. The photonic transmitter according to claim 11,wherein the second electrode lies directly on the second layer.
 18. Aprocess for fabricating a semiconductor photonic transmitter accordingto claim 11, the process comprising: producing a stack comprising asubstrate and the following layers successively stacked one on top ofthe other and each mainly lying parallel to a plane of the substrate: afirst layer disposed directly on the substrate comprising single-crystalsilicon encapsulated in a dielectric material, a second layer disposeddirectly on the first layer and comprising a dielectric material, and athird layer disposed directly on the second layer and comprising a III-Vgain material and a doped III-V crystalline material, wherein the III-Vgain material and the doped III-V crystalline material are encapsulatedin a dielectric material; producing a semiconductor laser sourceconfigured to generate an optical signal, the semiconductor laser sourcecomprising: a first waveguide made of silicon structured in thesingle-crystal silicon of the first layer, and a second waveguide madeof III-V gain material structured in the III-V gain material of thethird layer, the first and second waveguides being optically coupled toeach other by adiabatic coupling and being separated from each other bya first segment of the second layer, wherein in an interior of the firstsegment of the second layer, a thickness of the dielectric material isequal to a thickness of the second layer; and producing a phasemodulator produced on the substrate and configured to modulate anoptical signal generated by the semiconductor laser source, the phasemodulator comprising: a first electrode made of n-doped or p-dopedsingle-crystal silicon, a first portion of which is structured in thesingle-crystal silicon of the first layer, and a second electrode madeof doped III-V crystalline material structured in the doped III-Vcrystalline material of the third layer, a dopant of the secondelectrode being of opposite type to that of the first electrode, thesecond electrode being separated from the first electrode by a secondsegment of the second layer, wherein the producing of the stackcomprises producing the second layer with the following features: thethickness of the second layer is comprised between 40 nm and 1 μm, andin an interior of the second segment of the second layer, a thickness ofthe dielectric material is comprised between 5 nm and 35 nm, a rest ofthe thickness of the second layer in the interior of the second segmentbeing formed by a thickness of semiconductor material directly disposedon the first portion of the first electrode and which forms a secondportion of the first electrode, the thickness of semiconductor materialbeing chosen from the group consisting of a thickness of silicon, athickness of a SiGe alloy, and a superposition of thicknesses made ofsilicon and made of the SiGe alloy.
 19. The process according to claim18, wherein the producing of the second layer comprises: forming, on thesingle-crystal silicon of the first layer, a first sublayer made ofdielectric material of thickness equal to a thickness desired for thesecond portion of the first electrode, then in a location of the secondsegment of the second layer, producing a trench in the first sublayer, abottom of the trench opening onto the single-crystal silicon of thefirst layer, and not producing the trench in a location of the firstsegment of the second layer, then filling the trench with thesemiconductor material in order to form the second portion of the firstelectrode, a thickness of which is equal to the thickness of the firstsublayer, and then encapsulating, in a second sublayer made ofdielectric material, the formed second portion, a thickness of thesecond sublayer being equal to a thickness of the dielectric material inthe interior of the second segment.
 20. The process according to claim18, wherein the producing of the first waveguide of the semiconductorlaser source comprises: etching a layer of single-crystal silicon inorder to delineate, in the layer of single-crystal silicon, a centralportion of the first waveguide, and then forming, on the central portionof the first waveguide, an additional thickness of silicon in order tolocally increase a thickness of the first waveguide.